As mobile devices have been increasingly developed, and the demand for such mobile devices has increased, the demand for secondary batteries has also sharply increased as an energy source for the mobile devices. Among such secondary batteries is a lithium secondary battery having high energy density and high discharge voltage, on which much research has been carried out and which is now commercialized and widely used.
In terms of the shape of batteries, the demand for prismatic secondary batteries or pouch-shaped secondary batteries, which are thin enough to be applied to products, such as mobile phones, is very high. In terms of the material for batteries, the demand for lithium secondary batteries, such as lithium ion batteries and lithium ion polymer batteries, having high energy density, high discharge voltage, and high power stability, is very high.
The size and thickness of electronic devices have been increasingly reduced so as to satisfy liking of consumers. For this reason, the reduction in size and thickness of secondary batteries is also required so as to maximize space utilization. Therefore, it is necessary to provide various shapes of the secondary batteries corresponding to the shapes of the electronic devices and, in addition, to efficiently utilize the internal space of each of the electronic devices.
In particular, in recent years, designs of the electronic devices have played a very important role when consumers choose the electronic devices. For this reason, various designs are provided in place of a flat type design. For example, electronic devices, such as mobile phones and laptop computers, may be designed such that the electronic devices can be curved in an ergonomic manner.
A plurality of curved electronic devices has been developed and commercialized; however, most of the manufactured batteries are configured in a flat shape. As a result, space utilization is low, and it is difficult to mount batteries in the respective electronic devices due to such low space utilization. When external impact is applied to the electronic devices, therefore, the batteries may freely move in the respective electronic devices, with the result that the batteries may be damaged.
In order to mount a battery in an electronic device configured such that a battery mounting region, in which the battery will be mounted, is curved, therefore, it is required to curve the battery such that the battery is stably mounted in the curved battery mounting region of the electronic device.
Some technology for curving an electrode assembly has been proposed. For example, US Patent Application Publication No. 2007/0059595 disclosed a battery having a jelly-roll type electrode assembly configured such that a section of the electrode assembly perpendicular to a winding axis is curved. According to this disclosure, the curve is formed through thermal pressing using a concave heater and a convex heater.
Also, Japanese Patent Application Publication No. 1999-307130 discloses a method of thermally pressing a stack type electrode assembly using two rolls having different diameters to manufacture a battery configured in a curved structure.
According to the above disclosures, however, the electrode assembly is directly thermally pressed, with the result that deterioration of the battery occurs, which is very serious. In addition, the former technology is applied to the winding type electrode assembly (jelly-roll), with the result that larger stress occurs at the middle part of the electrode assembly on which stress is concentrated, and therefore, shape deformation, such as twisting, of the electrode assembly is great. On the other hand, the latter technology is applied to the stack type electrode assembly, with the result that, when the electrode assembly is pressed using the heating rolls, some electrode plates are nonuniformly pushed due to shear stress, and therefore, a short circuit may occur at opposite ends of the electrode assembly.
Furthermore, the above technology curves the electrode assembly, and therefore, it is necessary to curve a battery case in the same manner. Also, when an electrolyte is injected in to the battery case in a state in which the curved electrode assembly is mounted in the battery case, stress immanent in the electrode assembly during curving the electrode assembly is restored by a plasticizing action of the electrolyte, and therefore, the ends of the electrode assembly are pressed by the inside of the battery case, with the result that a possibility of a short circuit is increased.
Therefore, there is a high necessity for a secondary battery (battery cell) which is gently curved in the axial direction thereof, thereby solving the above-mentioned problems.